The present invention relates to a current output circuit, and more particularly to a current output circuit for use in a digital-to-analog converter.
Along with technological development, the request for high quality signal transmission progressively increases. Hence, how to design a digital-to-analog converter with high accuracy becomes an important issue.
Please refer to FIG. 1A which is a schematic circuit block diagram illustrating a conventional current-type digital-to-analog converter. The current-type digital-to-analog converter includes an encoder 10 and a current output circuit 11 including plural current output cells 111. When digital signal data[0:N] are inputted into the digital-to-analog converter, the digital signal data[0:N] are encoded into switch control signals via the encoder 10. The switch control signals control respective switches of the current output cells 111 to output analog voltages equivalent to the digital signal data.
Please refer to FIG. 1B which is a schematic circuit diagram illustrating one of the current output cells of FIG. 1A. PMOS transistor MP1 is considered as a constant current source under a bias VB1, and the current output path is determined by control signals C and CB wherein the signal CB has a reverse phase compared to the phase of the signal C.
As known, the design of the current output cell in the digital-to-analog converter will affect the linearity of output current. For example, the current output linearity of the current output cell can be increased by serially connecting thereto a transistor under a common voltage to enhance output impedance. The above way is generally feasible under a high operational voltage. However, with the increasing demand of reduced operational voltage, it is difficult to increase the output impedance and thus the current output linearity in the above-mentioned way.
Therefore, the purpose of the present invention is to develop a current output circuit for use in a digital-to-analog converter to deal with the above situations encountered in the prior art.
An object of the present invention is to provide a current output circuit for use in a digital-to-analog converter, which has a satisfying output impedance and current output linearity under a relatively small operational voltage.
According to an aspect of the present invention, there is provided a current output circuit for use in a digital-to-analog converter. The current output circuit includes a current source for providing a driving current, and a first output circuit coupled with the current source. The first output circuit includes a first metal-oxide semiconductor (MOS) transistor device having a source electrode thereof connected to the current source in series, a first voltage amplifier coupled between the source electrode and a gate electrode of the first MOS transistor device for keeping a voltage of the source electrode substantially constant, and a first controlled switch coupled between an operational voltage and the gate electrode of the first MOS transistor device for being switched ON or OFF in response to a first digital control signal, and allowing the driving current to be outputted from a drain electrode of the MOS transistor device when the first controlled switch is switched ON.
Preferably, the first digital control signal is generated by encoding a digital signal via an encoder.
For example, the first MOS transistor device can be a P-channel MOS transistor device.
Preferably, the current output circuit further includes a second output circuit. The second output circuit coupled with the current source includes a second metal-oxide semiconductor (MOS) transistor device having a source electrode thereof connected to the current source in series, a second voltage amplifier coupled between the source electrode and a gate electrode of the second MOS transistor device for keeping a voltage of the source electrode substantially constant, and a second controlled switch coupled between an operational voltage and the gate electrode of the second MOS transistor device for being switched ON or OFF in response to a second digital control signal, and allowing the driving current to be outputted from a drain electrode of the MOS transistor device when the second controlled switch is switched ON. The second digital control signal is preferably generated by encoding a digital signal via an encoder. For example, the second MOS transistor device can be a P-channel MOS transistor device.
Preferably, the first digital control signal received by the first output circuit has a reverse phase compared to a phase of the second digital control signal received by the second output circuit.
According to another aspect of the present invention, there is provided a current output circuit for use in a digital-to-analog converter. The current output circuit includes a current source for providing a driving current, a first output circuit and a second output circuit. Both first and second output circuit are coupled with the current source. The first output circuit includes a first metal-oxide semiconductor (MOS) transistor device having a source electrode thereof connected to the current source in series, a first voltage amplifier coupled between the source electrode and a gate electrode of the first MOS transistor device for keeping a voltage of the source electrode substantially constant, and a first controlled switch coupled between an operational voltage and the gate electrode of the first MOS transistor device for being switched ON or OFF in response to a first digital control signal, and allowing the driving current to be outputted from a drain electrode of the MOS transistor device when the first controlled switch is switched ON. The second output circuit includes a second metal-oxide semiconductor (MOS) transistor device having a source electrode thereof connected to the current source in series, a second voltage amplifier coupled between the source electrode and a gate electrode of the second MOS transistor device for keeping a voltage of the source electrode substantially constant, and a second controlled switch coupled between an operational voltage and the gate electrode of the second MOS transistor device for being switched ON or OFF in response to a second digital control signal, and allowing the driving current to be outputted from a drain electrode of the MOS transistor device when the second controlled switch is switched ON. The first digital control signal received by the first output circuit has a reverse phase compared to a phase of the second digital control signal received by the second output circuit.
According to a further aspect of the present invention, there is provided a current output circuit for use in a digital-to-analog converter. The current output circuit includes a current source for providing a driving current, and at least one output circuit coupled with the current source. The at least one output circuit includes a metal-oxide semiconductor (MOS) transistor device having a source electrode thereof connected to the current source in series, a voltage amplifier coupled between the source electrode and a gate electrode of the MOS transistor device for keeping a voltage of the source electrode substantially constant, and a controlled switch coupled between an operational voltage and the gate electrode of the MOS transistor device for being switched ON or OFF in response to a digital control signal, and allowing the driving current to be outputted from a drain electrode of the MOS transistor device when the controlled switch is switched ON.